Bingo, DavidY...you da man! In fact, the Lenovo instructions used the letter "l" and the number "1" inconsistently. It said (in Courier font):
8. Now create the USB key by entering this command: I had already used the correct letter for my USB drive (e.g., F instead of I). But making that one correction did the trick, and I was able to create the USB key.
BootableGenerator.exe --create -l -u -f [letter is used here]
*<-1> is the new name of the USB key (limit of 11 characters) [numeral is used here]
*<-u> is the drive letter assigned to the USB key
*<-f> is the name of the image
If the USB key has been assigned as Drive "I" by Windows, the image you downloaded is "UEFI_Bootable_v02.00.04.zip", and you name the USB key as "boot", the command would read as follows:
UefiBootableGenerator.exe --create -l boot -u I -f "UEFI_Bootable_v02.00.04.zip" [letter is used again here]
After making the necessary changes in the BIOS setup, I restarted the PC with the USB key. But the diagnostic tool doesn't seem to take over. Instead, it goes into AutoRepair mode again, then to "Diagnosing your PC", and again to "Scanning and repairing driver..." - same as before. (Lenovo Support said there was a known issue with this model, that they were unable to run the bootable diagnostic tool from a USB. So even though you helped me successfully create the USB key (which was better than the Support supervisor could do!), for some reason this laptop won't allow the diagnostic tool on the USB to take over.
If you have any further thoughts, I'd appreciate it!